Hard disk drives include a stack of magnetically coated platters that are used for storing information. The magnetically coated platters are mounted together in a stacked position through a spindle which may be referred to as a platter stack. The platter stack is typically rotated by a motor that is referred to as a spindle motor or a servo motor. A space is provided between each platter to allow a read/write head or slider to be positioned on each side of each platter so that information may be stored and retrieved. Information is stored on each side of each platter and is generally organized into sectors, tracks, zones, and cylinders.
Each of the read/write heads or sliders are mounted to one end of a dedicated suspension arm so that each of the read/write heads may be positioned as desired. The opposite end of each of the suspension arms are coupled together at a voice coil motor to form one unit or assembly that is positionable by the voice coil motor. Each of the suspension arms are provided in a fixed position relative to each other. The voice coil motor positions all the suspension arms so that the active read/write head is properly positioned for reading or writing information. The read/write heads or sliders may move from at least an inner diameter to an outer diameter where data is stored. This distance may be referred to as a data stroke.
Hard disk drives also include a variety of electronic circuitry for processing data and for controlling its overall operation. This electronic circuitry may include a pre-amplifier, a read channel, a write channel, a servo controller, a motor control circuit, a read-only memory (ROM), a random-access memory (RAM), and a variety of disk control circuitry to control the operation of the hard disk drive and to properly interface the hard disk drive to a system bus. The pre-amplifier may contain a read pre-amplifier and a write pre-amplifier that is also referred to as a write driver. The pre-amplifier may be implemented in a single integrated circuit or in separate integrated circuits such as a read pre-amplifier and a write pre-amplifier or write driver. The disk control circuitry generally includes a separate microprocessor for executing instructions stored in memory to control the operation and interface of the hard disk drive.
Hard disk drives perform write, read, and servo operations when storing and retrieving data. Generally, a write operation includes receiving data from a system bus and storing the data in the RAM. The microprocessor schedules a series of events to allow the information to be transferred from the RAM to the platters through the write channel. Before the information is transferred, the read/write heads are positioned on the appropriate track and the appropriate sector of the track is located. The data from the RAM is then communicated to the write channel as a digital write signal. The write channel processes the digital write signal and generates an analog write signal. In doing this, the write channel may encode the data so that the data can be more reliably retrieved later. The digital write signal may then be provided to an appropriate read/write head after first being amplified by the pre-amplifier.
In a read operation, the appropriate sector to be read is located and data that has been previously written to the platters is read. The appropriate read/write head senses the changes in the magnetic flux and generates a corresponding analog read signal. The analog read signal is provided back to the electronic circuitry where the pre-amplifier amplifies the analog read signal. The amplified analog read signal is then provided to the read channel where the read channel conditions the signal and detects "zeros" and "ones" from the signal to generate a digital read signal. The read channel may condition the signal by amplifying the signal to an appropriate level using automatic gain control (AGC) techniques. The read channel may then filter the signal, to eliminate unwanted high frequency noise, equalize the channel, detect "zeros" and "ones" from the signal, and format the digital read signal. The digital read signal is then transferred from the read channel and is stored in the RAM. The microprocessor may then communicate to the host that data is ready to be transferred.
The read channel may be implemented using any of a variety of known or available read channels. For example, the read channel may be implemented as a peak detection type read channel or as a more advanced type of read channel utilizing discrete time signal processing. The peak detection type read channel involves level detecting the amplified analog read signal and determining if the waveform level is above a threshold level during a sampling window. The discrete time signal processing type read channel synchronously samples the amplified analog read signal using a data recovery clock. The sample is then processed through a series of mathematical manipulations using signal processing theory to generate the digital read signal. There are several types of discrete time signal processing read channels such as a partial response, maximum likelihood (PRML) channel; an extended PRML channel; an enhanced, extended PRML channel; a fixed delay tree search channel; and a decision feedback equalization channel.
As the disk platters are moving, the read/write heads must align or stay on a particular track. This is accomplished by the servo operation through the use of a servo controller provided in a servo control loop. In a servo operation, a servo wedge is read from a track that generally includes track identification information and track misregistration information. The track misregistration information may also be referred to as position error information. The position error information may be provided as servo bursts and may be used during both read and write operations to ensure that the read/write heads are properly aligned on a track. As a result of receiving the position error information, the servo controller generates a corresponding control signal to position the read/write heads by positioning the voice coil motor. The track identification information is also used during read and write operations so that a track may be properly identified.
Hard disk drive designers strive to provide higher capacity drives that operate at a high signal-to-noise ratio and a low bit error rate. To achieve higher capacities, the areal density of the data stored on each side of each platter must be increased. This places significant burdens on the hard disk drive electronic circuitry. For example, as the areal density increases, the magnetic transitions that are used to store data on the platters must be physically located more closely together. This often results in intersymbol interference when performing a read operation. As a result, the hard disk drive electronic circuitry must provide more sophisticated processing circuitry that operates at higher frequencies to accurately process the intersymbol interference and the higher frequency read signal. In some cases, the spindle motor speed is increased which further increases the frequency of the read signal and the write signal. Furthermore, the increase in areal density requires that the servo control system be provided with a higher bandwidth to increase the read/write head positioning resolution.
As a consequence, additional electrical noise is often introduced into the hard disk drive system which may decrease the signal-to-noise ratio and increase the bit error rate. The higher frequency read and write signals are also more susceptible to noise than the lower frequency read and write signals present in lower capacity hard disk drives. More specifically, in higher capacity hard disk drives, noise is introduced into the analog read signal during read operations between the read/write heads and the read pre-amplifier, and between the read pre-amplifier and the read channel. For example, any noise that is introduced into the system between the active read/write head and the read pre-amplifier will be amplified by the read pre-amplifier. Similarly, noise is introduced into the analog write signal during write operations between the write channel and the write pre-amplifier, and between the write pre-amplifier and the read/write heads. Overall, this results in a decrease in the signal-to-noise ratio and an increase in the bit error rate. Other problems also arise due to the distance between the read/write heads and the pre-amplifier. For example, the distance between the pre-amplifier and the read/write heads introduces undesirable capacitive effects.